At present, in the coating of paper and board, two alternative methods and devices are commonly used, i.e. a blade coater or a bar coater. The present invention is expressly related to bar coaters, which have proved excellent especially in the surface-sizing technique. A bar coater is employed in coating and surface-sizing for spreading and smoothing the coating agent onto the surface of a paper or board web, to which the coating agent is introduced by means of a spreading roll.
In film size presses, the coating agent may also be introduced directly onto the faces of the rolls in the size press, from which, in the roll nip, it adheres to the web that passes through the nip. Coating bars currently in use ar usually made of steel. Coating bars are frequently provided with chromium plating to increase their service life. Bars with fully smooth faces are not used. Rather the bar face is provided with grooves, or steel wire may be wound onto the bar to form a solution similar to grooves on the bar face.
When the coating agent is applied by means of such a grooved coating bar, e.g., onto the rolls in the size press, the areas of the roll ends placed outside the web must be scraped clear by mean of lateral doctors. Otherwise the coating agent that remains in the end areas of the rolls causes considerable splashing in the roll nip.
The scraping of these lateral areas of the rolls has proved very difficult.